Research On Reliability Problems And Technical Countermeasures For Assessment And Retrofit Of Existing Masonry Buildings

At present,there are still many problems that need to be solved in the assessment and retrofit of existing masonry buildings.The reliability problems such as: 1)the determination method of target reliability indices for the assessment and retrofit of existing buildings,2)the resistance and load probability models suitable for the assessment and retrofit of existing buildings,3)the correction methods of the partial factors for the assessment and retrofit of existing masonry buildings and so on,need to be urgently studied.Focusing on the reliability problems,theoretical and experimental studies have been carried out,and corresponding technical countermeasures have been proposed.The main research contents and results of the thesis are as follows.(1)To address the problem of target reliability indices in the assessment and retrofit of existing masonry buildings,the differences in uncertainty characteristics between existing and new structures are analyzed,and the comprehensive determination method of target reliability indices for the assessment and retrofit assessment and retrofit of existing buildings is proposed based on the “life safety criterion,” “acceptable risk criterion,” “cost optimization criterion” and “reliability calibration method,” and the recommended values of target reliability indices for the assessment and retrofit of existing masonry buildings are determined.The results show that the target reliability index for the assessment of conventional masonry buildings may be taken as 3.4,and the target reliability index for retrofit may be taken as 3.7.(2)The assessment procedure of existing structures and the design procedures of new structures are mutually inverse.In contrast,the retrofit procedures of existing structures and the design procedures of new structures are similar.Considering the above facts,this thesis aims to solve the problem of the values of the partial factors in the assessment and retrofit of existing masonry buildings.Firstly,the correction probability models of resistance and load suitable for the assessment and retrofit of existing structures are proposed based on the Bayesian theory and the principle of equal probability of exceedance.Secondly,the correction methods of the partial factors of the assessment and retrofit of existing masonry buildings are established.Thirdly,the recommended values of the partial factors of the assessment and retrofit of existing masonry buildings are determined.Finally,the classification methods of the different subsequent working years of the safety assessment and seismic assessment of existing buildings are unified.The results show that: 1)In the case where the safety assessment of an existing masonry structure needs to be carried out,the material-partial factors for Class A,Class B,and Class C may be taken as 1.35,1.5,and 1.6,respectively by considering the truncated distribution characteristics of the resistance probability model and other characteristics;the permanent action partial factor can be taken as 1.0 by considering the quasi-constant distribution characteristics of the permanent load probability model;and the variable action partial factors may be taken as 1.3,1.4 and 1.5 respectively by considering the Gumbel distribution characteristics of variable action and other characteristics.2)In the case where the retrofit of an existing masonry structure will be performed,the material-partial factors for Class A,Class B,and Class C can all be taken as 1.6;the permanent action partial factors can all be taken as 1.3;and the variable action partial factors can be taken as 1.3,1.4,and 1.5,respectively.3)The calibration of reliability indicators shows that the reliability index of the assessment and retrofit according to the above partial factors may be ensured to be about 4.0,not less than 3.7,which meets the requirements of the current standard GB 50068-2018.(3)In response to the inconsistency between the seismic load capacity verification expression of cement mortar surface layer reinforced wall sections given by GB 50702-2011 and that given by JGJ 116-2009,the pseudo-static test data of cement mortar surface layer reinforced brick walls are analyzed,the resistance calculation formula for ultimate limit state verification of bearing capacity of structural members is proposed,and the deformation limits for different performance levels(Immediate Occupancy,Life Safety,and Collapse Prevention)are established.(4)From the perspective of reliability,the difference in seismic performance evaluation results caused by different calculation formulas of cement mortar surface reinforcement wall sections given by GB 50702-2011 and that by JGJ 116-2009 was analyzed.A simplified method for reliability assessment of existing masonry buildings is proposed to determine the seismic reliability indices of existing low-strength masonry building members pre-retrofit and post-retrofit.The results show that the seismic reliability index of unreinforced brick walls is about 2.2,and the seismic reliability index of cement mortar surface layer reinforced brick walls can reach2.5～3.1.(5)To study the overall reliability of existing masonry buildings pre-retrofit and post-retrofit,nonlinear analysis methods for masonry structures are examined and explored.Simulation analysis for the quasi-static test of low-strength brick walls was carried out by using the fiber elements and shear resilience constitutive models.From the perspective of seismic vulnerability,the seismic performances and reliability levels of an existing 4-story brick masonry building pre-retrofit and post-retrofit of reinforced mesh cement mortar surface layer are analyzed and studied.The results show that: 1)the fiber elements and shear resilience constitutive models can effectively reflect the seismic mechanical properties of masonry brick walls;2)the retrofit method of cement mortar surface layers may significantly improve the seismic performance of low-strength brick masonry structures and reduce the seismic vulnerability.(6)Based on the above research results,a technical procedure of assessment and retrofit taking into account the uncertain characteristics of existing masonry buildings is established.This will become a specific reference for the assessment and retrofit of existing masonry buildings in the future,and will provide related background materials for the subsequent revision of relevant standards.